Gyratory type crushers are used in the mining industry for reducing ore to a predetermined size for further processing. These style of crushers have taken over most large hard-ore and mineral-crushing applications which has made them an integral part of the mining industry. Typically, a gyratory crusher comprises a stationary conical bowl (or mortar) which opens upwardly and has an annular opening in its top to receive feed material. A conical pestle, opening downwardly, is disposed within the center of the bowl. The pestle is eccentrically oscillated for gyratory crushing movement with respect to the bowl. The conical angles of the pestle and bowl are such that the width of the passage decreases towards the bottom of the working faces and may be adjusted to define the smallest diameter of product ore. The oscillatory motion causes impact with the pestle and bowl, as a piece of ore is caught between the working faces of the bowl and pestle. Furthermore, each bowl and pestle includes a liner assembly replaceably mounted on the working faces, these liners define the actual crushing surface.
The pestle is formed by the liner, called a mantle fitted around the outside of a main shaft. The mantle provides a replaceable wearing surface. A threaded section on the shaft (or a threaded sleeve fit over the shaft) above the tapered portion of the shaft is provided for receiving a head nut. The head nut forces the mantle downward onto the tapered portion of the shaft, and is forceably tightened against the top of the mantle. Tightening prevents relative rotational movement between the head nut and the mantle. When the crusher is put into operation, the large forces involved in crushing stone cause a differential rotational movement between the shaft and the mantle. The head nut on the threaded section of the shaft is also caused to rotate relative to the shaft, in a direction which acts to further tighten the head nut onto the mantle. Thus, the rotational movement of the head nut relative to the shaft causes a large force to be transmitted in a downward direction from the head nut so as to forceably wedge the mantle onto the tapered portion of the shaft, securing the mantle to the shaft. The force also causes the bottom surface of the head nut to be pressed tightly against the top surface of the mantle such that the frictional force between the head nut and the mantle is quite large.
The frictional force between the head nut and the mantle makes it difficult to loosen the head nut by turning. Additionally, during operation of the crusher the crushing surface of the mantle is subjected to a hammering action by repeated impact of the rock or other material being crushed. This hammering action causes the working surface of the mantle to expand by cold working. The expansion of the mantle works to increase the fictional force between the head nut and the mantle. The cumulative effect of the tremendous frictional force between the head nut and the mantle is that it becomes impossible to loosen the head nut by turning it.
It is, however, necessary to remove the head nut when the mantles become worn and need replacing. Since it is not practical to loosen the head nut by turning, it must be cut from the threaded section of the shaft (as with a cutting torch). Removing the head nut in this manner damages the head nut beyond repair so that it cannot be used again. The threaded section of the shaft (or sleeve) is also easily damaged when removing the head nut in this fashion, such that the threaded shaft must be repaired, or possibly replaced. Thus, the cost associated with removing the head nut to replace worn mantles becomes excessive.
A solution to this problem proposed in prior art is to provide a burning ring between the mantle and the head nut. The burning ring is adapted so as to engage to the upper surface of the mantle and the lower surface of the head nut. When the mantle is being replaced, the burning ring is cut with a cutting torch, relieving the frictional forces bearing on the head nut. The threaded portion of the head nut may then easily be unscrewed from the shaft and the mantle can be removed.
The main method taught in prior art of affixing the burning ring to the head nut as well as the burning ring to the mantle is using keying systems. Keys are placed between the surfaces of the head nut and burning ring and between the head nut and the mantle. Typically, the keys are inserted between the components of the head nut assembly (head nut, burning ring, top of mantle) after the components are mounted on the main shaft. A common method is to form a semicircular slot running radially on each of the interfacing component surfaces, align the slots, and then place a circular pin into the slots so as to couple the surfaces together. Other shapes of slots or grooves are also used in conjunction with a key or bolt inserted after the slotted surfaces are aligned. With this arrangement, the key must be welded to the interfacing components in order for the key to be held in place. Only small welds are possible, since large welds would be on or near the exposed crushing surfaces. If the welds are on the crushing surface they are subject to breaking, allowing the key to come loose.
Other methods of attachments utilize the key as the "cutting piece." The "cutting piece" is cut by the operator to separate the components. All of these methods require that the key be exposed to the interior of the crusher. Using exposed keys to connect the head nut, burning ring and mantle is problematic, since the interior of the crusher is a harsh environment which very often results in the keys being knocked out from between the components, uncoupling the components.
If the interface between the head nut and the burning ring or the interface between the burning ring and the mantle become uncoupled, the self-tightening feature of the head nut is lost, since the mantle no longer transfers the twisting force (which occurs when being impacted by rock) to the head nut. The mantle can loosen from the main shaft. If the gyratory crusher is not shut off, the free spinning mantle can cause extensive damage to the crusher. The mantle may crack or break, requiring replacement, or the mantle may twist with respect to the shaft, and gouge the shaft. Alternatively, the mantle may move vertically along the shaft, causing damage to the head nut or the threads of the head nut. All of these can result in extensive repairs at a great cost and with long machine downtime. At the very least, the separation of the assembly components make it necessary to turn off the machine, remove the crushing material and replace the connection, which requires a good deal of labor and lost time.
Additionally, installation problems arise when the components must be aligned after they are mounted to the shaft of the crusher to accommodate a key. The problems occur because the head nut assembly must be tightened to prevent excess "play" between the components. The key cannot be placed between the faces of the components when too much space exists between the components. When the head nut is torqued to the proper level, the slots in the component faces may not line up to accommodate the key. The operator must then untorque the head nut, realign the components, and re-torque the head nut until the correct alignment is attained.
An alternative connecting method depicted in prior art shows the coupling of the head nut to the burning ring by welding. Although welding forms a tight bond between the components and eliminates alignment problems, distortion of the head nut can result. Distortion is caused by the heat required to weld the head nut to secure the head nut to the burning ring and also to plasma torch cut the welds to free the head nut from the burning ring. Distortion of the head nut prevents the head nut from easily being removed from the shaft, and reused. Instead, the head nut must be cut off and replaced, eliminating any advantage gained by using the burning ring in conjunction with the head nut.